1. Field of the Invention
The present invention relates to a secondary battery charge/discharge control apparatus and method with an active balancing circuit and algorithm to charge and discharge a plurality of secondary batteries connected in series.
2. Description of the Related Art
Large-capacity lithium secondary batteries used in electric vehicles each have a relatively low operating voltage, for example, a voltage of about 2 volts (V) to about 4.7 V, and a relatively high operating current, for example, a current of about 25 amperes (A) to about 50 A.
In an electric vehicle, tens to hundreds of large-capacity lithium secondary batteries are connected and used. In the process of manufacturing a battery assembly, a battery of a unit module (for example, a single battery, or a few batteries are connected in parallel or in series) needs to be charged and discharged at maximum capacity. However, there are several technical difficulties, such as low conversion efficiency for converting electric energy to an alternating current (AC), and for recycling the electric energy.
In an apparatus for simultaneously charging and discharging a large-capacity secondary battery in a switch mode, a single lithium secondary battery with an operating voltage of about 2 V to about 4.7 V may be discharged. Electricity obtained by the discharging may need to be regenerated in a nationwide AC grid by consuming the electricity as heat, or by converting the electricity to AC electricity.
A direct current (DC)-to-AC (DC/AC) converter used to regenerate power may have extremely low conversion efficiency, for example typically 40%, since a voltage of a single battery is considerably lower than a voltage of a commercial AC grid (for example, 220 V or 380 V). During conversion, remaining energy may be consumed as heat in the DC/AC converter.
Accordingly, a process of using a conventional secondary battery charge/discharge apparatus is known to use the most power in a factory of manufacturing batteries including conditioning equipment for cooling. When secondary batteries having slightly different capacities and slightly different internal resistances are connected in series and are charged and discharged by a single bidirectional constant current power supply, it may be difficult to charge and discharge each of the plurality of secondary batteries at a constant voltage (CV) due to a difference in characteristics of the secondary batteries, and overcharge or overdischarge may occur.
A technical configuration associated with a battery pack control module and a method of using the same for balancing of a plurality of lithium secondary batteries or groups of lithium secondary batteries is disclosed in Korean Patent Publication No. 10-2011-0008337. Additionally, a technical configuration including discharge route circuits for each secondary battery connected in series and for protecting overcharge is disclosed in Korean Patent Publication No. 10-2010-0122911. However, in that disclosure, because each of the plurality of secondary batteries includes a separate discharge resistor and a switching element to form a discharge route circuit, power consumption, and manufacturing costs may increase.